In free space optical communication systems, diode lasers are attractive as a source of light because of their small size and relatively high efficiency. A diode laser, in general, is a body of a semiconductor material which when biased emits light through the recombination of oppositely charged carriers. The body generally includes regions of opposite conductivity type, which when properly biased, inject charge carriers of one conductivity type from one of the regions into the other where the light generating recombination occurs. Diode lasers in general include a cavity which confines both the charge carriers and the generated light. Also, at the ends of the cavity the light is reflected in a manner so as to generate a beam of substantially coherent light which is generally emitted from an edge of the diode.
Since most single-emitter diode lasers have large angular beam divergences, generally greater than 10.degree., large collimation telescopes are necessary to form the appropriate communications beam, which is typically about 20 cm. in diameter. This large beam diameter is required for free space optical links because typical propagation distances are on the order of 10,000 km. and the diameter of the 35 receiving telescope is about 10 cm. in diameter. These constraints dictate that the optical transmitter for such a link must have a power output of 500 mW or more in a narrow divergence diffraction limited single lobed far field. Stable single spectral and spatial mode operation are also required, and this stability must be maintained at multi-GHz modulation rates. Single element diode lasers operating in a single mode are limited to operating powers of about 100 mW. Therefore, suitable coherent diode laser arrays which contain a plurality of the diode lasers are necessary to provide the desired power and radiant intensity of the beam which is needed for free space optical communication systems.
A type of diode laser array which has been recently developed is a coherent monolithic array of injection-coupled grating surface emitting (GSE) diode lasers. An array of GSE diode lasers, in general, comprises on a single substrate of a semiconductor material a plurality of electrically isolated gain sections that are within a network of distributed Bragg reflector (DBR) passive waveguides. The gain sections contain regions of opposite conductivity type and a cavity in which the charge carriers of opposite conductivity type are formed and recombine to generate light. The DBR sections function in a three-fold manner. The second grating order of the DBR provides wavelength selection and feedback necessary to support laser oscillation. In first order, the grating couples light out normal to the substrate surface. Finally, the transmissivity of the DBR sections are sufficiently large so as to couple adjacent gain sections. It has been found that GSE arrays can have output powers in excess of 1
W, far field beam divergence of 1.degree. by 0.01.degree. and spectral outputs narrower than 0.25 Angstroms. One such DBR array is shown and described in the article of G.A. Evans et al entitled "Coherent, monolithic two-dimensional (10.times.10) laser arrays using grating surface emission", published in APPLIED PHYSICS LETTERS, Vol 53(22) Nov. 28, 1988, pgs. 2123-2125. However, for optical communication purposes, it is necessary to be able to switch this type of array at relatively high speeds. Therefore, it is desirable to have GSE array which not only can provide high power output, but which can be switched relatively fast for communication purposes.